The use of wireless communication systems is very widespread and continues to increase. As the number of wireless users increases, the likelihood increases that a user will have a wireless device, such as cellular telephone, available when a need arises to make an emergency call such as a 911 call. Landline systems detect a user's location when an emergency call is made. Such detection is easy, because a landline telephone call is made from a fixed location, typically with the telephone number originating the call being associated with the location. Location detection for wireless telephone calls is much more difficult, but is extremely important. For example, it may be critically important to able to identify the location of a user who may be injured or disoriented and in urgent need of assistance, but does not know the location or is unable to communicate it to an operator.
A number of solutions have been considered. One of the simplest solutions would be the use of global positioning system (GPS) technology, because much of the required infrastructure is already in place. A wireless unit equipped with a GPS receiver could easily adapt the information provided by the GPS receiver to provide location information to an emergency call system, provided that the GPS receiver was able to compute the position data.
However, reliance on the GPS system alone to provide location data is problematic in emergencies. A GPS receiver must have a clear view of several satellites in order to obtain a position fix, and the receiver may take several minutes, even with an acceptable view, to obtain the fix. In emergencies, reliability and speed in obtaining location information can be critical. A user's location must be determined as quickly as possible, so that assistance can be provided promptly. Presently proposed standards for location identification for wireless devices contemplate prompt location identification. These standards require a worst case speed for emergency location identification that is significantly faster than the worst case speed accepted by users of GPS systems.
If a network is properly designed, fast and reliable location identification can be accomplished using triangulation of signals received from a number of base stations in different locations. However, a wireless device must be in sight of and in range of at least four base stations in order to consistently accomplish location identification in this manner. Prior art network design concentrates primarily on positioning base stations and configuring the base station antennas so as to maximize coverage and reduce interference. A wireless unit can communicate successfully with a network if it is within sight and range of one base station. Deployment of base stations so that a unit is within range of more than one base station results in an overlap of coverage areas and some increase of interference between base stations. When considered by themselves, these outcomes are undesirable.
Therefore, prior art design of networks does not impose any requirement that a wireless unit will be within range of more than one base station, and the generation of a network design that is suitable for identifying the location of a wireless unit occurs either accidentally or as a result of field tests and adjustments. Traditionally, networks are designed using geographic and user information, with the base station locations being predetermined primarily according to business considerations. The antenna configurations are optimized to maximize coverage, using tools that yield values for antenna azimuth, transmit power and tilt. If it is desired that the network so designed be usable for location determination, actual field tests are then conducted. One or more users move around within the service area, noting the points at which they are able to obtain location information. This data is then used to make adjustments to the antenna configurations to improve location determination capabilities. Further field tests are then conducted, followed by additional adjustments, and so on. Needless to say, this procedure is both complex and costly.
There exists, therefore, a need for systems and techniques for wireless network design that can be implemented at the initial planning stages of a network and that take into account the need to identify the location of a wireless unit making an emergency call and optimize the design of the network so as to allow for identification of the location of a mobile unit without excessive interference and without a need to deploy an excessive number of base stations.